The debrisoquine/sparteine genetic polymorphism is due to mutant alleles of CYP2D6. Individuals lacking production of this cytochrome P450 are unable to metabolize over 30 drugs. In several instances, lack of metabolism of a drug results in toxicity or exaggerated responses. About 7.5% of Caucasians possess two mutant alleles of CYP2D6. In order to characterize these mutant genes and in an effort to develop simple polymerase chain reaction-based diagnostic tests, a mutant CYP2D6 allele was directly cloned from leukocyte DNA taken from an individual that was unable to metabolize debrisoquine. The gene was completely sequenced and found to contain a G to A transition that alters the consensus 3 prime splice site at the junction of intron 3 and exon 4 of CYP2D6. A simple non-radioactive PCR test was developed, based on this single mutation, that can be used to screen individuals by analyzing leukocyte DNA for presence of mutant P450 genes. This mutant allele, designated CYP2D6(B), represents about 75% of all mutant CYP2D6 genes. A second mutant CYP2D6 gene, designated CYP2D6(C), was identified that produces a protein lacking a single amino acid. This enzyme is catalytically active but is not stably expressed in human liver. A polymerase chain reaction-based test was also developed for CYP2D6(C). This is a rare allele representing less than 2% of all mutant genes. Complementary DNA expression systems have been developed to evaluate the ability of CYP2D6 to metabolize drugs and activate chemical carcinogens. A lymphoblastoid cell containing the CYP2D6 CDNA was constructed and used to demonstrate metabolic activation of a carcinogen found in tobacco smoke.